Any monitor species (even the smaller ones, like an ackie) require a more specialized husbandry setup then what you described you are ready for. Blue tongue skinks are fun, and once you have them dialed in they are easy to care for. Any of the Indo species require a slightly different setup (higher humidity) then the Australian Species. Someone else already mentioned a Uromastyx. They require pretty high basking temperatures, but not super difficult. Other lizards I could think of would be Agamas and Chuckwallas. You mentioned that they had to be ok in a 4ft cage. Do you already have the cage or is that just your size requirement? If you already have it what type of cage is it and what are the actual dimensions. Lizards are more active than most snakes, and will make use of the entire cage, so width and height of the cage is just as important.